AI Preview Localized Rollback

Goal

Replace full-document AI preview snapshotting with localized insert-mode rollback so preview accept/cancel only touch the preview block range, while preserving current undo/redo and selection behavior.

Checklist

• [completed] Read current localized stream path, accept path, and preview tests
• [completed] Add failing tests for localized preview ownership and rollback
• [completed] Implement localized preview state and marker-based range handling
• [completed] Run targeted verification for touched packages and tests
• [blocked] Run check, then create or update the PR

Verification

• bun test packages/ai/src/lib/transforms/aiStreamSnapshot.spec.ts packages/ai/src/lib/transforms/undoAI.spec.ts apps/www/src/__tests__/package-integration/ai-chat-streaming/streamHistory.slow.tsx
• pnpm install
• pnpm turbo build --filter=./packages/ai --filter=./apps/www
• pnpm turbo typecheck --filter=./packages/ai
• pnpm build
• pnpm turbo typecheck --filter=./packages/ai --filter=./apps/www after root build
• pnpm lint:fix
• agent-browser open http://localhost:3100/blocks/editor-ai

Blocker

• pnpm check still fails outside this diff in existing fast AI chat tests:
  • packages/ai/src/react/ai-chat/hooks/useAIChatEditor.spec.tsx
  • packages/ai/src/react/ai-chat/hooks/useEditorChat.spec.tsx
  • packages/ai/src/react/ai-chat/utils/getLastAssistantMessage.spec.ts
• The failure is a SyntaxError from missing exports in packages/plate/src/react/index.tsx, so PR creation is blocked by repo rules.

Agreed Design

• Keep the public surface on tf.ai.*
• Optimize insert-mode preview only
• Keep preview state private and editor-scoped
• beginPreview accepts { originalBlocks }
• selectionBefore is still captured inside beginPreview
• Preview-owned content is tracked by transient top-level block markers
• cancelPreview restores the exact original block slice plus selection
• acceptPreview localizes cleanup/commit and never rebuilds the whole document
• Untouched blocks should keep identity because preview accept/cancel stop using full-doc setValue

Open Implementation Notes

• Mark preview-owned top-level blocks with aiPreview: true
• Do not mark the aiChat anchor block as preview-owned
• If preview starts by removing an empty paragraph, capture that exact block in originalBlocks
• If preview inserts after existing content, capture []
• Derive the current preview range from contiguous marked top-level blocks
• If preview range is invalid/non-contiguous, fall back safely instead of silently guessing